1. Field of the Invention
The present invention relates to an automatic focal point control apparatus suitable for optical equipment such as cameras.
2. Related Background Art
U.S. Pat. No. 4,447,719 describes a conventional automatic focal point control apparatus used in a camera. A relative deviation between a pair of object images formed by two light beams passing through two different portions of the exit pupil of a photographing lens is photoelectrically detected by two one-dimensional image sensors. Focus detection of the photographing lens is performed according to the detected deviation. The two one-dimensional image sensors have light-receiving surfaces and generate time-serial analog outputs corresponding to the light intensity distributions of the light-receiving surfaces, respectively ("light" here includes light components such as infrared rays excluding the visible light components). The time-serial analog outputs are calculated according to predetermined processing to obtain time-serial defocus signals. The defocus signal is defined as a signal representing a difference between an object image focal plane of the photographing lens and a prospective focal plane (a plane optically equivalent to the film surface), i.e., a signal representing a distance. The defocus signal consists of an absolute value and a sign (corresponding to a direction of focusing position corresponding to a prospective focal position). These defocus signals are multiplied with a predetermined weighting coefficient to obtain a weighted average thereof. The photographing lens is controlled and driven to focus the image according to the weighted average.
In a conventional automatic focal point control apparatus of this type, noise components in the time-serially generated defocus signals can be averaged and reduced to provide stable operation.
According to such a conventional automatic focal point control apparatus of this type, however, a sole weighting coefficient is multiplied with a plurality of defocus signals. When the object moves or changes, the sole weight coefficient is unsatisfactory to achieve accurate focal point control. For example, a user holds a camera and fails to keep the camera in the same position (e.g., the camera is accidentally moved during the shutter release), or the user intentionally pans the camera to change the object. In this case, a plurality of time-serially generated defocus signals correspond to different objects at the respective timings. The signals include coarse defocus signals corresponding to objects of low luminance or contrast subjected to relatively poor focal point detection and objects of high luminance or contrast subjected to relatively good focal point detection. When the defocus signals of different reliability levels are weighted by the sole weighting coefficient, the defocus signal of low reliability level is excessively weighted. The weighted average of the defocus signals then has poor reliability. As a result, when the photographing lens is driven or display is performed on the basis of such a weighted average, stable operation tends not to be guaranteed.